Implantable stimulation devices generate and deliver electrical stimuli to nerves and tissues for the therapy of various biological disorders, such as pacemakers to treat cardiac arrhythmia, defibrillators to treat cardiac fibrillation, cochlear stimulators to treat deafness, retinal stimulators to treat blindness, muscle stimulators to produce coordinated limb movement, spinal cord stimulators to treat chronic pain, cortical and deep brain stimulators to treat motor and psychological disorders, occipital nerve stimulators to treat migraine headaches, and other neural stimulators to treat urinary incontinence, sleep apnea, shoulder sublaxation, etc. Implantable stimulation devices may comprise a microstimulator device of the type disclosed in U.S. Patent Application Publication 2008/0097529, or larger types of stimulators such as spinal cord stimulators or pacemakers for example.
Microstimulator devices typically comprise a small, generally-cylindrical housing which carries electrodes for producing a desired electric stimulation current. Devices of this type are implanted proximate to the target tissue to allow the stimulation current to stimulate the target tissue to provide therapy. A microstimulator's case is usually on the order of a few millimeters in diameter by several millimeters to a few centimeters in length, and usually includes or carries stimulating electrodes intended to contact the patient's tissue. However, a microstimulator may also or instead have electrodes coupled to the body of the device via a lead or leads. A multi-electrode microstimulator 10 having a single anode (Ea) and several selectable cathodes (Ec1 et seq.) in shown in FIG. 1. Further details regarding such a microstimulator 10 can be found in the above-referenced '529 application.
Implantable microstimulators 10 are typically powered by an internal battery, which periodically needs to be recharged. Such recharging is usually accomplished by an external charger, which produces a magnetic field to ultimately induce a current in a coil in the implant. This induced current is rectified, and used to charge the implant battery.
Recharging the implant battery by magnetic induction works well, and allows the implant battery to be charged wirelessly and transcutaneously (i.e., through the patient's tissue). However, such techniques also suffer from heat generation. In particular, the external charger can heat up, and if it gets too hot may burn the patient.
The inventors have noted that this problem of external charger overheating can be exacerbated if the external charger itself requires recharging. In this regard, note that the external charger may too contain a rechargeable battery, whose power is used to produce the magnetic field to charge the implant's battery. If the external charger's battery needs recharging, this provides an additional heat load on the external charger, particularly if the external charger's battery and the implant's battery require recharging at the same time. The inventors believe that a solution to this problem of excessive heating in an external charger is therefore indicated, and this disclosure provides solutions.